This invention relates to gas turbine engine exhaust nozzles and, more particularly, to cooling systems for use therein.
The high velocity imparted to the exhaust gases of a gas turbine engine by the exhaust nozzle provides the thrust for propulsion. This thrust is substantially parallel with, and opposite to the direction of, exhaust gases exiting the nozzle. Consequently, if the direction of the exhaust gases is changed, the direction of propulsive thrust is correspondingly varied. Typically, aircraft gas turbine engines are provided with nozzles which are fixed in the axial direction and the aircraft maneuvering is accomplished solely by airframe control surfaces.
Advanced aircraft configurations contemplate, and may even require, the selective redirection (or vectoring) of gas turbine engine thrust in order to enhance aircraft performance and to provide the aircraft with operational characteristics heretofore deemed impractical. Copending patent applications, Ser. No. 572,340, filed Apr. 28, 1975, "Flight Maneuverable Nozzle For Gas Turbine Engines," -- Nash et al, and Ser. No. 572,341, filed Apr. 28, 1975, "Actuating Means For A Thrust Vectoring Gas Turbine Engine Exhaust Nozzle," -- Nash, which are assigned to the same assignee as the present invention and the disclosures of which are incorporated herein by reference, teach a flight maneuverable exhaust nozzle and means for actuating same which will efficiently and practically alter the direction of gas turbine engine exhaust nozzle gases. Effective integration of a thrust vectorable gas turbine engine within a vertical/short take-off and landing (V/STOL) aircraft requires a gas turbine engine which achieves high thrust levels to avoid the weight penalties associated with auxiliary lift engines and which would be used only during the take-off and landing flight phases. To achieve such high lift thrust levels, high temperature augmentation (afterburning) is required, and one key to a successful engine is an effective cooling scheme for the V/STOL exhaust nozzle.
Particularly severe cooling problems are characteristic of V/STOL exhaust systems for augmented operation. In particular, a conventional augmented exhaust system (no V/STOL capability) experiences only minor changes in pressure and velocity along the exhaust stream flow path. This means that efficient film cooling can be achieved by use of constant-area slots or holes for injecting the cooling film, with only minor changes in coolant flow rate and distribution resulting as the operating conditions are varied. By comparison, the V/STOL exhaust systems of the type described in the aforementioned copending patent applications have severe gas stream pressure variation and cooling flow control problems. These systems are efficient in the lift mode due to the variable flow path geometry, the throat being rotated with the deflector so that the gas flow is turned upstream of the throat and velocities substantially lower than sonic velocity. Serious pressure losses are thus avoided and efficient performance results. This feature, while benefiting performance, results in difficult coolant flow control since the changes in flow path geometry produce wide variations in velocity and pressure of the hot gas along the flow path. Thus, a means is needed to regulate V/STOL nozzle cooling flow under the varying exhaust stream pressure conditions. Adequate cooling flow in the lift mode must be provided when flow path cross-sectional areas are large, velocities low and exhaust stream static pressures high. Adequate, but not excessive, coolant flow must also be provided when operating in the cruise mode with smaller flow path cross-sectional areas, high velocities and correspondingly low exhaust stream static pressures.
Furthermore, cooling of the surface which turns or deflects the hot exhaust stream is difficult since, due to the impingement which results from turning the exhaust flow, the exhaust stream pressure is as great or greater than the fan air pressure which is the preferred source of coolant. This condition precludes use of film cooling and, in general, prevents coolant from flowing from the relatively low pressure, cool side of the deflector liner to the relatively high pressure, hot side. An effective, reliable means is required for cooling the critical flow deflecting surface.
In addition, a means is needed for supplying an adequate cooling supply to the rotatable deflector in such a way that the coolant flows only when operating in the V/STOL mode. By terminating coolant flow when the deflector is stowed in the cruise position, maximum exhaust system efficiency and aircraft cruise range is assured.
In short, the need for the coolant flow control is very important for two reasons: first, there is much more area to be cooled in the V/STOL mode than in the cruise mode and thus more coolant flow is required; secondly, the exhaust stream gas pressure is generally higher in the lift mode since all of the flow path is upstream of the nozzle throat. This means that the film coolant pressures must be correspondingly high. In the cruise mode, however, the throat is located further forward and the gas stream pressures are consequently lower. If coolant pressure is not reduced when in the cruise mode, excessive cooling flows will result with severe performance penalties. Additionally, other hot surfaces upstream of the nozzle (such as turbine blades and vanes) may be starved of coolant with resultant overheating.